That line of code let firstObject = allObjectLists[0].valueForKey("useItems") is pulling out the useItems object, which is a List object. This is why it's reporting there's no method named toDictionary() available for it.

If you're trying to get the first object in useItems to generate a dictionary off that, it should be:

let allObjectLists = realm.objects(UseItemList.self) // Get all 'UseItemList' objects from Realm as a `Results` object
let firstObjectInList = allObjectLists.first! // Get the first UseItemList object from the 'Results' object
let useItemsInFirstObject = firstObjectInList.useItems // Access the 'useItems' List object in the first object
let firstUseItem = useItems.first! // Access the first item from the 'useItems' List object
let toDict = firstItem.toDictionary() // Convert the first item into an array

Obviously you can condense this down into one line of code, but you need to make sure you're accessing all of the elements in the right order or else you won't get to a proper Object at the end. :)